A/n: I had so much fun writing this, especially the last two scenes. I hope Senku isn't oc. PLEASE, PLEASE, PLEASE READ IT WHILE LISTENING TO LOVE (wave to earth).
SYNOPSIS: For years, you’ve thought that Senku Ishigami’s actions were for efficiency, logic, and science. You dismissed any ideas that suggested otherwise. Because, for you, that was just how Senku was. Until one handmade lip stain forces you to question the gestures you grew to normalize. They were just acts of science, right?
TAGS: fluff, fluff, and more fluff.
WC: 4704
The first person to vocally point it out was Yuzuriha. Her eyes were wide with curiosity and bewilderment. You nearly laughed in disbelief.
Because from your perspective, what she was saying barely made any sense. This was the Senku Ishigami she was talking about. The science-obsessed boy who approached everything in life with logic.
“You're telling me that's…normal?” She gestured at the small milk carton in your hands that Senku gave you minutes ago.
You nodded casually while sipping. “Yeah, it's just milk.”
You said it so simply, as if you were ignoring the details of what happened earlier that clearly contradicted your “it's just milk”.
The moment the third period came to an end, Senku dragged you out of class without saying anything. You had grown used to it, hence you let him tug on your wrist softly without asking questions.
He guided you swiftly through the bustling hallway. The two of you reached his locker, and before you could peek to check what he was reaching out for, he handed you a small milk carton. It was still cold.
He clicked the roof of his tongue out of annoyance. “Your glucose levels crashed halfway through the third period. How many times do I have to tell you not to skip breakfast?”
You gave him a cheeky smile, already familiar with the gesture. “Thanks, Sen. I woke up late again, okay?”
Behind the two of you, Taiju and Yuzuriha watched the interaction. Both were clearly shocked, however, for two completely different reasons.
Taiju aggressively pointed at the milk carton in your hands, “Senku! How did you keep it cold?!”
“Insulated water bottle and ice. She won't drink it unless it's cold.” He said so casually while reading something on his phone.
Yuzuriha, however, wasn't curious about the science behind the cold milk carton. No, she was curious about the scientific explanation as to why you acted as if this was normal. How Senku acted as if this was normal.
“Was that really normal? I don't think it was…” she asked when it was just the two of you; her voice remained soft and polite.
Senku had made his way to the science laboratory while Taiju was eager to see another fascinating experiment of his.
You hummed, “Yeah, that's just how Senku has always been. I call it his acts of science. He says it's to keep me efficient.”
Senku always did things like that. He had a knack with his small gestures, weaving them into the fabric of your daily life.
It wasn't strange or new to you. Not really. Not when he's been consistently doing it for years.
⋆⭒˚.⋆
You constantly complain about headaches almost every afternoon, especially during study sessions. Muttered complaints fell from your lips easily, and rubbing your temples became a habit.
“You're dehydrated.” You heard him say from across you while working with a circuit board, one that was too complex for you to understand. It wasn't a question; it sounded like a statement from his observation.
You lift your head up, tired eyes meeting his striking red ones. With a weak smile, you muttered out, “Thanks for the diagnosis, doc.”
“Tch.”
He didn't say anything else as he pulled out a water bottle from his bag, followed by a pack of electrolyte powder.
He pushed the opened bottle forward, the bits of electrolyte powder already dissolving into the water. “You’re inefficient when your body is malfunctioning.”
You gave him a knowing smile before he continued to work on his circuit board. Senku's focus returned to it, or so you thought.
“And your caffeine intake is garbage. Stop consuming it like a lifeline.”
Your smile faltered. “Hey, you're acting like you don't drink coffee too, genius!” You retorted, frustration lacing your tone.
Without even glancing up from his circuit board, he shot back, “Unlike someone, I don't rely on it to function properly.”
You threw an eraser at him, to which he dodged effortlessly, a smirk tugging the corner of his lips.
Still, despite your banter, there was an undeniable truth. Senku always seemed to notice everything before you did.
Yet you never asked. That was just the way he was—always too observant. Always a step ahead.
Right?
⋆⭒˚.⋆
The science laboratory always smelled like chemicals, metals, and something burning whenever Senku felt like it.
It was enough to remind students passing by that Senku Ishigami was inside—most likely taking something apart or fixing it, mixing chemicals, or accidentally blowing something up.
The laboratory wasn't just occupied by him; there were multiple other students working on something. Though they were never as over-the-top as the ones Senku worked on.
In the middle of it all sat Senku, who had recently cleaned up after a chemical experiment. The sleeves of his lab coat were now rolled, revealing his forearms as he worked on the blueprint of his next rocket model.
The room around him hummed with conversation, though most people knew better than to interrupt him while he worked.
Which was why a question from another student caught everyone off guard.
“Hey, Senku.”
“What?” He replied flatly.
The student hesitated briefly before pointing at Senku's wrist. “What's with the hair tie?”
Every head in the science laboratory turned immediately because now that someone was brave enough to mention it, there really was a black hair tie looped around his wrist.
It wasn't unusual at first. Not until people started noticing months ago that Senku always had one on his wrist. They assumed it belonged to him, perhaps to tie his hair during experiments—but no, his hair defied the laws of physics.
And he never used it. Not even once.
The student leaned back against the table, a grin on his face as he asked, “You got a girlfriend or something?”
The room instantly erupted as the question flew out of the students’ mouths.
“No way! Senku? Seriously?”
“As if! You really think he has any time for romance?”
“There's no way, are we talking about the same Senku Ishigami?”
“That's what a hair tie means, though!” The student argued defensively, pointing at Senku's wrist. “It's literally a thing, guys wear them when they're taken.”
Senku didn't even bother to look up from the blueprint he was working on, though an irritated look was evident on his face.
“Dumbest thing I've heard all day.”
“So that's a no, Senku?”
“Tch, idiots. Use your brains for once.”
That unfortunately didn't answer anything. However, everyone seemed to be too interested and eager in the conversation.
“So is it someone's?” A student from a corner asked.
Senku finally looked up from his blueprint, clicking the roof of his tongue. “Obviously. Why would I use one when my hair defies gravity and is never in the way?”
The room went silent before,
“OHHHH?”
“No way?!”
Senku looked far more annoyed after hearing their reactions, which, unfortunately, only encouraged everyone further.
“It's basic efficiency." He deadpanned. “Unlike you idiots, I account for predictable patterns to avoid unnecessary situations.”
Then, the doors of the science laboratory suddenly opened.
“Senku?” Your voice cut through the room of chaos, completely unaware of the chaos you've interrupted and caused.
Everyone turned to the door, not just Senku.
You blinked, feeling uncomfortable at the unexpected attention from everyone. “Why is everyone looking at me like that?”
“No reason,” half of the room replied in unison before pretending to focus on their work again.
You frowned suspiciously, eyes settled on Senku as your facial expression asked him questions. He didn't say anything, just stared back as you walked towards him.
You stopped next to him and allowed the suspicion to simmer down as you remembered the reason you were there. “Do you have a hair tie?”
Without saying a word, he lifted his wrist toward you, as if by muscle memory. Your fingers moved automatically and reached out for it.
“Lost another one? You're too careless, idiot.”
You ignored his remark, and with one smooth motion, you slid it off his wrist before tying your hair with practiced ease.
“Thanks, Sen.” You gave him a grin. “I'll see you after school.”
“Hm.”
Senku returned to his blueprint without saying anything else the moment you left.
The room stayed silent for three seconds after you closed the door behind you before erupting into chaos once more.
“What was that?!”
“I knew it was her!”
Senku ignored their loud chaos, his hand already reaching out for the pocket of his pants, and pulled out another hair tie, slipping it onto his wrist.
“Tch, you people are seriously annoying.”
⋆⭒˚.⋆
You never questioned Senku's acts of science. Never. Not even when Yuzuriha’s teasing became a little too frequent.
It was normal; you were used to it for years. Why bother to question it now?
The sound of rain suddenly falling from the skies sliced through your reverie. You halted, staring at the building window next to the staircase, before a groan escaped from your lips.
“Great. Just great.” You had forgotten your umbrella. Although you always forget to bring it, this time was different. You had asked Senku to go home without waiting for you. The same guy who never forgets to bring an umbrella and checks the weather forecast.
You made your way down the stairs, and your mood was a perfect match with the gloomy weather outside.
“Took you long enough.”
Your head snapped up toward the familiar voice, and your gaze immediately landed on him, Senku. One hand in his pocket while leaning against a wall near the staircase.
Your mouth fell open in shock. “Sen—?! What are you doing here?!”
“I was waiting for you, what else?” He answered casually. Too casually, as if you didn't ask him not to wait for you.
He walked closer to you, his lab coat hanging on one shoulder while his bag hung on the other. An umbrella settled on one of his hands.
“I told you not to wait!”
“And?”
“It's raining, and it's late!”
“Yeah, no kidding. I'm not blind.”
Without waiting for your response, he threw the lab coat on your shoulder, catching you off guard again. “Wear it, you'll get cold if you don't.”
You frowned, yet slipped on his lab coat. “You should've gone home.”
“Yeah, and leave you here with a 10 billion percent chance of being stranded? You never bring an umbrella or check the weather forecast, idiot.” His tone was completely matter-of-fact, like the answer should've been obvious.
He watched as you struggled with the sleeves. He clicked the roof of his tongue before walking closer, hands already hovering over yours.
“You're hopeless.” He spoke with a tone that was softer than before. Then, with quick, efficient movements, his hands adjusted the sleeves and collar of his lab coat.
Your breath got caught in your throat for a brief moment because of the proximity. “You really didn't have to wait for me, y’know?” Your voice sounded small.
“You get sick too easily. You have the immune system of a Victorian child.” The tone of his voice remained soft as he buttoned the coat properly, ensuring it was secured around your body.
You laughed at the last part, eyes admiring the way his hands moved with the same precision he had when working on an experiment.
He took a step back after he was done. "Besides, Byakuya would never let me hear the end of it if he found out I left without you.”
You grinned, already hooking your arm with his. “Yeah, he would probably say he raised you better than that. Let's go home, Sen.”
That evening, the two of you walked home through the wet streets and drizzling rain. Yet you remained dry. Senku's lab coat protected you from the raindrops as well as the umbrella he held tightly above the two of you.
You never said anything regarding the proximity or the gesture. You were used to him acting like this. It was normal in your vocabulary.
⋆⭒˚.⋆
Your mother's loud knocking on your bedroom door finally woke you up from your slumber.
“Aren't you hanging out with Senku today? Why are you still sleeping?” She asked as she entered your room, eyeing your sleepy figure, who had just woken up.
“Hang out with Senku…?”
Your eyes shot open in realization. You were supposed to help Senku purchase an engine component for his new rocket model today.
You immediately got out of bed, nearly tripping over your blanket, and ran straight to the shower, one of your hands grabbing your towel on the way there.
“Be careful!” Your mother yelled out of concern.
After a shower, you scrambled to get ready. Your clothes were everywhere after you forgot to prepare an outfit the night before. After selecting one, you searched for a pair of socks before rushing downstairs.
“Senku's already outside.” Your mother said as she watched you hastily put on your socks, hands already reaching out for your sneakers.
You bid your mother goodbye, already making it past the front door. Your eyes spot Senku, who was waiting for you.
He turned right before your hands fell to your knees for support, panting and out of breath after you practically ran while getting ready.
“You should consider using an alarm.” His voice was laced with amusement as he knelt down, hands already reaching out to tie your shoelaces.
“You could've tripped over your shoelaces again.”
Your heart skipping a beat didn't help you at all. It was normal, you swore to yourself. He fixes your shoelaces all the time. It was just efficient.
Your breathing finally returned to normal. “I did, I slept through it.”
He chuckled before standing up, hand reaching out to flick your forehead. “Idiot, did you sleep late again?”
You raised a brow, hitting him playfully. “You're one to talk, genius.”
⋆⭒˚.⋆
Senku's experiments were a constant source of fascination. You were always interested and could hardly take your eyes away whenever he was mixing his colorful array of chemicals in the lab.
“You're too close,” he said, voice edged with concern.
“Huh?”You replied, momentarily distracted.
Before you could process his warning any further, you felt Senku's hand grab your wrist, pulling you behind him right before the Erlenmeyer flask erupted, bubbles frothing violently at the surface.
Your eyes widened in curiosity as you peeked from behind him. “Whoa,”
His grip never left yours, never loosened; it remained steady around your wrist. “Exactly why I told you not to stand too close.”
You couldn’t help but chuckle softly, “You say that every single time, Sen.”
“And you're stubborn every single time.” He countered back, his grip tightening around your wrist, ensuring you wouldn’t inch any closer.
His grip felt firm, protective, and undeniably comforting. It felt like a silent assurance that you were safe in his presence. You didn't question it; after all, Senku always did things like that, didn't he?
⋆⭒˚.⋆
One of the perks of living near Senku meant you could always crash at his place, and Byakuya allowed you to do so. Especially whenever he's busy training to be an astronaut on the other side of the world.
The living room of the Ishigami household was lit by the television screen across you, as well as the faint light coming from the windows. Senku's presence was near and grounding as the two of you watched another move that you picked, of course.
You didn't realize you were crying—not at first. The dialogues bled with a soundtrack that was clearly designed to hurt you slowly.
Beside you, Senku exhaled through his nose. He couldn't comprehend why you kept choosing movies that clearly tugged at your heartstrings.
“Statistically speaking, you've chosen emotional damage 6 times in a row.” He spoke, voice flat as he stared at the screen with a look of boredom. He wouldn't be watching these movies if it weren't for you.
“Shut up, Sen.” You laughed through your flowing tears. Your voice was shaky as the soundtrack was starting to affect you harder.
That was when he finally looked at you, a frown appearing on his face as he saw the tears streaming down, your hands already hovering above your eyes to rub them.
You were crying again. Rubbing your eyes on top of that, too.
“Stop doing that.” He said immediately, tone firm.
You blinked, “Huh?”
“Tch,” his hands reached out, pulling yours away from your face before you could even process it. His grip was gentle, quick, and precise.
“You’re going to irritate your corneas if you keep rubbing your eyes,” he continued, as if a doctor was scolding their patient. “Damage delicate blood vessels, make the inflammation worse— do you want me to continue?”
He warned, and you shook your head. “I'm just—”
“Crying and rubbing your eyes.” He finished for you, clearly unimpressed.
His hands never released your wrists. With a sigh, he pulled them down gently. “Why do you always choose movies that do this to you? It's stupid.”
You opened your mouth, ready to argue and defend your choices. “It's part of the experience, Senku.”
“Yeah, right, I believe you,” he mocked softly.
He shifted closer, hands releasing your wrists. And with the same irritating precision he used for everything else, he reached out for your cheeks, gently wiping your tears away with the pad of his thumbs.
His touch was careful, almost too careful. As if it were another adjustment made to a system that had slightly fallen out of equilibrium.
“The movie wasn't even great,” he muttered, yet his thumb didn't stop. “Crying over a predictable plot outcome?”
You glared at him with angry tears, yet he smiled in return as you lunged to attack him.
“Stop watching them with me if you're only going to judge me, you menace!”
His chuckle filtered the dialogue from the movie, letting you tug at his hair all you want.
“Who will stop you from damaging your eyes if I stopped?” He countered casually—too casually, as if it was a natural response.
You halted, your hands frozen in his hair for a moment before he gently pried them away.
“What?” He asked, red eyes already staring into yours, the light from the window behind you reflecting on it. “You're acting like I don't do it often.”
Right. He does. He does it so often that it has always felt natural. And somewhere along the way, you stopped noticing it entirely. Perhaps you’ve noticed, yet dismissed the thoughts. Never allowing yourself to look further into what those actions actually meant.
Because it was Senku.
Senku Ishigami, the genius who treats inconveniences as problems to solve for the sake of efficiency. However, he was also the boy who remembered things nobody else did.
And still, despite everything, you never called it anything else. It was just Senku's acts of science. You chose to dismiss the feeling of your chest tightening painfully as the realization was coming closer.
No, it was just his acts of science, right?
⋆⭒˚.⋆
Wrong.
You were wrong, utterly wrong. Or at least you thought so, as the realization refused to be dismissed any further after being buried from years of excuses, the familiarity of routines, and the comfort that you understood Senku's nature.
Because that was easier. It was easier than confronting the terrifying realization that maybe everyone else had been right all along.
Perhaps it was never purely about science. Maybe it wasn't just about efficiency, logic, or the fact that Senku disliked being unable to solve inconveniences.
Maybe—
Your breath got caught in your throat again, between your tempting realization and the reflex to push it down. The thought settled painfully; it felt almost impossible to make it go away.
Your memories weren't much help either, as they replayed endlessly without permission, refusing to stay harmless.
The reminders, the way he noticed things before you did, the way he was always prepared for your inconveniences as if he had already calculated them, the way he always remembered.
The realization was everywhere, taunting you dangerously. You could see it in everything you dismissed as his acts of science.
Which was why the sight of the small metal tube of lip stain on your hands felt almost unbearable as it weighed and catalyzed your realization.
“Try it already,” Senku spoke from his desk chair across from you, who sat on his bed, your usual spot. He sounded mildly annoyed at your silence.
You couldn't really blame him. You've been silent for too long, buried in your thoughts that refused to go quiet.
It was almost funny, really. A lip stain was all it took to catalyze your realization. However, it wasn't just any lip stain. It was a replica of the one that you've been searching for endlessly for the past few weeks.
The same lip stain you cried over with embarrassing levels of devastation after losing it, especially since it was discontinued everywhere. You could recall the countless times you brought it up whenever you were with Senku. How the shade was your favorite since it suited you so well, and how you couldn't find any alternatives.
“You…made this?” You spoke out softly, finally finding your voice again.
He clicked the roof of his tongue. “Yeah? Why are you acting so surprised as if I hadn't made things before?”
Your eyes refused to leave the lip stain in your hands. The metal surface felt cool against your fingers. You noticed the engraving of your name on the packaging, which wasn't like the packaging of your previous lip stain with stickers that wore out easily after a few uses.
You heard the wheels of his desk chair roll closer towards you. “I changed the packaging since you always complained about the lettering wearing off easily.”
“How did you remember the shade?” You looked up at him, and your eyes immediately met his again. He was already looking at you.
He gave you his signature laugh. “That was the hard part. Learning cosmetology was 10 billion percent easier. I had to rely on information and pictures online, as well as yours.”
“I would've finished it within a week if you had tested out the shade samples yourself.” He smirked, “Byakuya kept whining that it'd ruin the surprise.”
You tilted your head, “Byakuya knew about this?”
“Of course. He caught me watching a cosmetology crash course at 2 AM.”
You found yourself laughing at his response. Your grin was wide, so wide that it made your cheeks hurt. You were genuinely so happy.
You kept smiling even when you were no longer laughing. Senku raised a brow, mirroring the previous tilt of your head. “What?”
“You're—” your grin appeared again, your cheeks were really starting to hurt. “You're so sweet, Sen.”
He paused in disbelief. “Hah? What are you talking about—”
Your sudden hug caught him off guard, and your added weight pushed the desk chair backward slightly. Your arms were wrapped around his neck, one hand still gripping the lip stain tightly.
“Thank you, Sen. Seriously, I don't know how to thank you enough.”
He clicked the roof of his tongue again, acting annoyed, yet the smile on his face said otherwise. As well as his hand that steadied you against him, returning the hug.
“You can thank me by actually trying it. How else am I supposed to know I got the shade right?”
The grin on your face remained as you pulled away. “Okay,” your voice became strangely soft.
You were about to use your phone screen as a mirror when Senku reached out to grab the lip stain from you. “It has a built-in mirror. Less hassle.”
You stared at him in awe. “Oh my gosh, you really considered the design of the packaging.”
He shrugged, handing the lip stain back to you. “The original design was inefficient. You would need a separate mirror each time.”
He didn't say anything else as he watched you apply it to your lips. A satisfied smile formed on your face, and Senku nearly exhaled in relief. He didn't fail; he succeeded in reviving your favorite lip stain.
Your eyes shifted to meet his again, your smile refusing to leave your face. “How do I look, Sen?”
He paused for a second, “The same but with a different shade of lip color.”
You gasped and hit him playfully. “How rude! You learned cosmetology but not about how to compliment a girl?”
He shrugged, leaning back in his chair. “Why would I waste my time with the latter?”
You rolled your eyes before admiring the shade with the built-in mirror. “You could at least say I look prettier.”
“You always do,” he responded too quickly, without any hesitation in the tone of his voice. It was too straightforward, as if he were reciting a fact.
You short-circuit for a few seconds, cheeks burning up slightly as you look away.
“You're really amazing, Sen...” You mumbled, admiring his creation while trying to ignore the warmth from your cheeks.
“Yeah?” A quiet chuckle escaped from him.
“I've built rockets, and this is what impressed you the most?” He added, a teasing grin on his face.
“Well, this one is the most unexpected thing you would do, y’know?”
He shook his head with another chuckle. “You're welcome.” He moved himself using his chair, a hand reaching for a notebook on his desk. “I haven't tested the transfer resistance yet, but it stains better than the original.”
“Why not?”
“Do you expect me to wear it?”
You laughed out loud, “Wouldn't you do it for science?”
“Nice joke.” He deadpanned. “I did it for you, not for science, idiot.”
Somehow, that confession hit harder than anything else.
“The Senku Ishigami is doing something that isn't for science? Is the world meeting its doom soon?” you joked mockingly.
“That's worse than the previous one.” He deadpanned again.
You giggled, hands reaching out to pull his chair closer to you, a mischievous idea had already appeared in your head just seconds ago.
“What are you doing?”
You didn't answer. Instead, you tugged his head down, closer to your face. With a swift motion, you pressed a quick kiss on his cheek.
You pulled away, satisfied with your results. There was a clear mark from the gloss, yet no pigment. The only pigment you found was the burning ears of Senku, dusted with a faint red.
“Results concluded, it's transfer proof.” You grinned at his frozen state. “You can log the results in your notebook now.”
A teasing smirk formed on your face as you eyed his silent and flustered figure. “You good, Sen?”
“Yeah…” he muttered out after snapping out of his frozen state, voice low as he turned away from you to reach for a pen.
Oh, you were savoring every moment. That was your payback after he made you spiral over his acts of science.
Perhaps the two of you wouldn't talk about it further for now. However, you were—as Senku would say it—10 billion percent sure there's time for that in the future.
For now, you'll mainly appreciate his acts of science, or his way of showing that he cared far more than you thought.
⋆⭒˚.⋆
Bonus:
“Where did you learn how to make cosmetics, Senku-chan?” Gen’s curious question hung in the air, capturing Senku's attention.
The scene unfolded in front of the two of them, along with Suika and Soyuz, who were gathered around Amaryllis and you. Together, you were meticulously applying makeup on Kohaku’s face in preparation for the selection.
“It's chemistry, mentalist.” His gaze briefly flickered towards Gen and deadpanned.
Gen sweat dropped. “Yes, but I never expected you to touch that part of chemistry, Senku-chan…”
The scientist shrugged, dismissing the comment. And before Gen could question him further, you suddenly walked to Senku with a bright grin on your face.
“Sen, can you make my lip stain again? I've been missing it lately.” Your eyes sparkled with enthusiasm, something Senku could never say no to.
‘Ah.’ Gen's question had been answered the moment the words fell from your lips. The mentalist wasn't even surprised anymore. ‘So this is the so-called acts of science Yuzuriha-chan was talking about.’
a/n: I had so much fun writing this in one sitting. It was way longer than I initially planned because of the last scene. Thank you for reading, I hope you guys enjoyed this one <3.
